Method of controlling the stopping operation of vacuum pump and device therefor

ABSTRACT

A method for controlling the operation of a vacuum pump when stopping the gas transferring operation thereof, the vacuum pump having a housing in which a pump chamber is formed and a gas transferring body which is rotatably disposed in the pump chamber for transferring gas, the method comprises the steps of reducing rotational speed of the gas transferring body to a first preset speed below a second preset speed that is lower than a normal speed of the gas transferring body during normal gas transferring operation of the vacuum pump, maintaining the speed of the gas transferring body below the second preset speed, and stopping the rotation of the gas transferring body when the temperature of the housing reaches a predetermined temperature which is lower than that of the housing during the normal gas transferring operation of the vacuum pump.

BACKGROUND OF THE INVENTION

The present invention relates to a method of controlling the stopping operation of a vacuum pump which transfers gas by moving a gas transferring body in a pump chamber and also to a device for practicing the method.

A vacuum pump is used to discharge gas for use in semiconductor manufacturing processes from a process chamber and to create a vacuum environment in the process chamber. As the vacuum pump, a positive displacement vacuum pump is known which has roots type or screw type pump rotors as a gas transferring body. Generally, the displacement vacuum pump has a pair of pump rotors which is disposed in the pump chamber of a pump casing and a motor which drives the pump rotors to rotate.

Narrow clearances are formed between the pair of the pump rotors and between each rotor and the inner surface of the casing such that the pair of pump rotors rotates without contact with the casing, and the paired pump rotors are rotated synchronously in opposite directions. Such rotation of the pump rotors, causes gas in the pump casing to be transferred from the suction side to the discharge side of the casing, and then to flow out of the process chamber which is connected a suction port.

Gas used in semiconductor manufacturing processes may sometimes contain an element which is solidified during being transferred (hereinafter, such solidified element is referred to as “solid products”). Because the vacuum pump generates heat of compression during transferring of the gas, the vacuum pump in operation, or specifically the casing and the pump rotors of the vacuum pump, becomes relative high in temperature. While the vacuum pump maintains a high temperature, the casing and the pump rotors are thermally expanded. Thus, the clearance between the pump rotor and the inner surface of the pump chamber facing the pump rotor becomes wider and, therefore, solid products tend to get into the clearance and accumulate therein.

When the operation of the vacuum pump is stopped, the vacuum pump becomes gradually lower in temperature and the thermally-expanded casing and pump rotors contract, so that the clearance is narrowed, so that, the solid products accumulated in the clearance are held between the pump rotors and the inner surface of the pump chamber. When the vacuum pump is restarted, the pump rotors may be prevented from rotating by the solid products held between the pump rotors and the inner surface of the pump chamber and cannot be rotated by starting torque of a motor. If the vacuum pump cannot be restarted by the staring torque of the motor, a tool is engaged with a rotary shaft of the vacuum pump and then a torque is applied to the rotary shaft by manually rotating the pump rotors with the tool, thus removing the solid products from the clearance and making the vacuum pump ready for restarting.

Unexamined Japanese Patent Publication No. 2004-138047 discloses a method of starting a vacuum pump which enables the solid products to be removed without manual operation and the vacuum pump to be restarted. According to the method disclosed in the above publication, when restarting the vacuum pump having therein solid products, a torque is applied from the motor to the pump rotors for rotation in normal direction. Thereafter, the torque applied to the pump rotors becomes zero and again a torque is applied to the pump rotors for rotation in forward direction. Thus, torque is applied to the solid products accumulated between the pump rotor and the inner surface of the pump chamber. As a result, the solid products become brittle and are broken, so that they are removed from the clearance and, therefore, the vacuum pump can be started without manual operation.

According to the vacuum pump starting method disclosed in the above publication, the vacuum pump is started from a state that the solid products are held between the pump rotors and the inner surface of the pump chamber. Thus, solid products need to be removed from the clearance by application of a force of the pump rotors before it becomes possible for the vacuum pump to transfer the gas. If solid products are held tightly or a large amount of solid products is held between the pump rotors and the inner surface of the pump chamber, the pump rotors need to be rotated for many times for application of force that is enough to break the solid products, with the result that the time for prior operation of the vacuum pump before actual gas transferring will be lengthened. According to the starting method disclosed in the above publication, the vacuum pump requires a relatively long time before gas transferring becomes possible after the vacuum pump has been started.

The present invention, which is made in view of the above problems, is directed to a method and an apparatus of controlling the stopping operation of the vacuum pump. The method and apparatus of controlling the stopping operation of the vacuum pump according to the present invention prevents the vacuum pump from stopping in a state that solid products are held between the inner surface of the pump chamber and the gas transferring body and permits the vacuum pump to be restarted rapidly for transferring of gas.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for controlling the operation of a vacuum pump when stopping the gas transferring operation thereof, the vacuum pump having a housing in which a pump chamber is formed and a gas transferring body which is rotatably disposed in the pump chamber for transferring gas, the method comprises the steps of reducing rotational speed of the gas transferring body to a first preset speed below a second preset speed that is lower than a normal speed of the gas transferring body during normal gas transferring operation of the vacuum pump, maintaining the speed of the gas transferring body below the second preset speed, and stopping the rotation of the gas transferring body when the temperature of the housing reaches a predetermined temperature which is lower than that of the housing during the normal gas transferring operation of the vacuum pump.

In accordance with the present invention, a device is used for controlling the operation of a vacuum pump when stopping the gas transferring operation thereof. The vacuum pump has a housing in which a pump chamber is formed and a gas transferring body rotatably provided in the pump chamber for transferring gas. The device includes means for controlling the stopping operation of the vacuum pump and detection means for detecting the temperature of the housing and generating a detection signal when the temperature of the housing reaches a predetermined temperature. The predetermined temperature is lower than a temperature of the housing during normal operation of the vacuum pump when gas is transferred by the gas transferring body. The controlling means is operable to reduce rotating speed of the gas transferring body to a speed below a second preset speed that is lower than a normal speed of the gas transferring body during normal gas transferring operation of the vacuum pump in response to a pump-stop command signal for stopping the gas transferring body. The controlling means is also operable to stop the rotation of the gas transferring body in response to the detection signal from the detection means.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a plan cross-sectional view of a roots pump of a first embodiment according to the present invention;

FIG. 2(a) is a cross-sectional view taken along the line A-A in FIG. 1;

FIG. 2(b) is a cross-sectional view taken along the line B-B in FIG. 1;

FIG. 2(c) is a cross-sectional view taken along the line C-C in FIG. 1;

FIG. 3 is a graph showing change of rotation speeds of rotor and change of temperature of a rotor housing according to the first embodiment of method of controlling the stopping operation of the vacuum pump; and

FIG. 4 is a graph showing change of rotation speeds of rotor and change of temperature of a rotor housing according to a second embodiment of method of controlling the stopping operation of the vacuum pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe the first preferred embodiment of a method and apparatus of controlling the stopping operation of a roots pump as a vacuum

pump which transfers gas in a semiconductor manufacturing equipment with reference to FIGS. 1 through 3. The front and the rear of the roots pump in the following description are indicated by the double-headed arrow Y in FIG. 1.

Referring to FIG. 1, the roots pump 10 of the first preferred embodiment includes a multistage roots pump 11A having a plurality of sets of rotors as a gas transferring body and a single stage roots pump 11B having only one set of rotor and these roots pumps 11A and 11B are assembled integrally. Since the multistage roots pump 11A and the single stage roots pump 11B are different from each other only in the number of sets of rotors, the same reference numerals will be used to denote the same or similar elements or components and the description thereof will be omitted in the following description. FIGS. 2(a) through 2(c) show cross-sectional views of the multistage roots pump 11A and FIGS. 2(b) and 2(c) are provided without diagrammatic representations of a cooling passage.

As shown in FIG. 1, the multistage roots pump 11A and the single stage roots pump 11B include a front housing 13, a rotor housing 12, a rear housing 14 and a gear housing 33, respectively. The front housing 13 is fixed to the front end of the rotor housing 12 and a sealing body 36 is fixed to the front end of the front housing 13. The rear housing 14 is fixed to the rear end of the rotor housing 12 and the gear housing 33 is fixed to the rear end of the rear housing 14. The front housing 13, the rotor housing 12, the rear housing 14 and the gear housing 33 are made of ductile iron.

As shown in FIG. 2(b), the rotor housing 12 of the multistage roots pump 11A includes a cylinder block 15 and a plurality of partition walls 16. The partition walls 16 are also made of ductile iron. The cylinder block 15 includes a pair of blocks 17, 18 and each partition wall 16 includes a pair of wall elements 161, 162. As shown in FIG. 1, a space serving as a pump chamber 39 is formed between the front housing 13 and its adjacent partition wall 16 in the rotor housing 12 of the multistage roots pump 11A. Similarly, spaces serving as pump chambers 40 through 42 are formed between any two adjacent partition walls 16, and a space serving as pump chamber 43 is formed between the rear housing 14 and its adjacent partition wall 16, respectively.

Meanwhile, the rotor housing 12 of the single stage roots pump 11B has no partition wall 16 and it is made of a cylinder block 15 which includes a pair of blocks 17 (only one block 17 being shown. In FIG. 1). A space as a pump chamber 50 is formed by the front housing 13, the rear housing 14 and the cylinder block 15 in the rotor housing 12 of the single stage roots pump 11B.

Rotary shafts 19, 20 are rotatably supported in parallel relation to each other through radial bearings 21, 22, respectively in the front housing 13 and the rear housing 14 of the multistage roots pump 11A and the single stage roots pump 11B. The rotary shafts 19, 20 are positioned in the axial direction by being fixed to the rear housing 14 through the double row radial bearings 21, 22 positioned at the rear end of the multistage roots pump 11A and the single stage roots pump 11B.

A plurality of rotors 23 through 27 is formed integrally with the rotary shaft 19 of the multistage roots pump 11A. Similarly, a plurality of rotors 28 through 32, which is of the same number as the above rotors 23 through 27, is formed integrally with the rotary shaft 20 of the multistage roots pump 11A. The rotors 23 through 32 are of substantially the same shape or profile and size as viewed in the direction of axes 191, 201 of the rotary shafts 19, 20. The thicknesses of the rotors 23 through 27 become smaller in this order. Similarly, the rotors 28 through 32 are formed with thickness which become smaller in this order.

Two rotors 23 and 28 are accommodated in the pump chamber 39 so as to mesh with each other. Similarly, pairs of the intermeshing rotors 24 and 29, 25 and 30, 26 and 31, 27 and 32 are accommodated in the pump chambers 40, 41, 42, 43, respectively, so as to mesh with each other. Meanwhile, in the single stage roots pump 11B, rotors 51, 52 are formed integrally with the rotary shaft 19, 20, respectively. The rotors 51, 52 are accommodated in the pump chamber 50 so as to mesh with each other. The rotary shafts 19, 20 are made of ductile iron.

Small clearances are formed between the rotors 23, 28 and the front housing 13 and also between the rotors 23 through 32 and their adjacent partition walls 16, respectively. Similarly, small clearances are formed between the rotors 27, 32 and their adjacent opposite rear housing 14. That is, the clearances are formed between rotors 23 through 32 and the inner surfaces of the pump chamber in which the rotors 23 through 32 are accommodated. Therefore, the rotors 23 through 32 are rotatably arranged without contact with the inner surfaces of the pump chambers 39 through 43.

In normal operation, the rotary shafts 19, 20 are thermally expanded from the rear ends of the rotary shafts 19, 20 in connection with the radial bearings 21, 22 toward the front ends of the rotary shafts 19, 20. The rotor housing 12 and the rear housing 14 are also thermally expanded. Because the rotary shafts 19, 20 are higher in temperature than the rotor housing 12 and the rear housing 14, the rotary shafts 19, 20 are more thermally expanded than the rotor housing 12. Thus, rotors 23 through 32 are moved toward the front end of the multistage roots pump 11A in normal operation as compared to when the roots pump operation is stopped. The moving distances of the rotors 23, 28 located at the front of the rotors 23 through 32 are the largest of all the rotors 23 through 32. Therefore, the clearances in the pump chambers 39 through 43 are formed so as to be narrower in this order. That is, the clearance in the pump chamber 39 is the largest of all the clearances in the pump chambers 39 through 43.

The gear housing 33 is fixedly mounted to the rear housing 14 of the multistage roots pump 11A and the single stage roots pump 11B. The rotary shafts 19, 20 extend through the rear housing 14 into the gear housing 33. Gears 34, 35 are mounted for engagement with each other on the ends of the rotary shafts 19, 20 which extend into the gear housing 33. An electric motor MA is mounted to the gear housing 33 of the multistage roots pump 11A and an electric motor MB is mounted to the gear housing 33 of the single stage roots pump 11B, respectively.

Driving forces of the electric motors MA, MB are transmitted through a shaft couplings 44 to the rotary shafts 19 in the multistage roots pump 11A and the single stage roots pump 11B. Therefore, the rotary shafts 19 are rotated by the electric motors MA, MB in the direction of arrows R1 in the FIGS. 2(a) through 2(c). Then, the rotating force of the rotary shafts 19 is transmitted through the gears 34, 35 to the rotary shafts 20. The rotary shafts 20 are rotated in opposite direction from the rotary shafts 19, as indicated by arrows R2 in FIG. 2(a) through 2(c), and synchronously because of the intermeshing gears 34, 35 mounted on the rotary shaft 19, 20.

The rotors 23 through 27, 51 are rotatable in the direction of the arrows R1 by the driving force of the electric motors MA, MB and the rotors 28 through 32, 52 are rotatable in the direction of the arrows R2. Thus, the paired rotors 23, 28 are rotatable in opposite directions and synchronously with each other. Similarly, the pairs of the rotors 24, 29, the rotors 25, 30, the rotors 26, 31 and the rotors 27, 32 are rotatable in opposite directions and synchronously with each other. In addition, the rotors 23 through 32 rotate at the same rotation speed as the electric motor MA and the rotors 51, 52 rotate at the same rotation speed as the electric motor MB.

As shown in FIG. 2(b), a passage 163 is formed in each partition wall 16 of the multistage roots pump 11A which has an inlet 164 and an outlet 165. Thus, the any two adjacent pump chambers 39, 40, 41, 42, 43 are connected with each other through the passage 163 in the partition wall 16 between the above two adjacent pump chambers.

In the multistage roots pump 11A, an inlet 181 is formed through the block 18 for fluid communication with the pump chamber 39, as shown in FIG. 2(a). As shown in FIG. 2(c), an outlet 171 is formed through the block 17 for communication with the pump chamber 43. In the single stage roots pump 11B, an inlet and an outlet, are formed in the block 17 and the block 18, respectively, for communication with the pump chamber 50.

As shown in FIG. 1, the multistage roots pump 11A is connected through a supply passage 45 to the single stage roots pump 11B. Specifically, the outlet of the single stage roots pump 11B is connected through the supply passage 45 to the inlet 181 of the multistage roots pump 11A. In the single stage roots pump 11B of the roots pump 10, rotations of the rotors 51, 52 by the electric motor MB introduces gas through inlet into the pump chamber 50, and the rotation of the rotors 51, 52 transfers the gas from the outlet to the supply passage 45.

In the multistage roots pump 11A, when the rotors 23 through 32 are rotated by the electric motor MA, the gas flowed from the single stage roots pump 11B through the supply passage 45 is introduced into the pump chamber 39 through the inlet 181, and then the gas is transferred by the rotations of the rotors 23, 28 through the inlet 164, the passage 163 and the outlet 165 of the partition wall 16 into the pump chamber 40. Similarly, the gas is further transferred in the roots pump 10 from one pump chamber to another while being compressed gradually. The gas is compressed to the maximum pressure in the pump chamber 43 and discharged out of the roots pump 10 through the outlet 171.

Referring to FIG. 2(a), a cooling device 54 is disposed on the top surface of the rotor housing 12 or the block 18 and another cooling device 55 is disposed on the bottom surface of the block 17. The cooling device 54 is connected to a supply pipe 541 and a discharge pipe 542. Similarly, the cooling device 55 is connected to a supply pipe 551 and a discharge pipe 552. Coolant from a coolant supply source T is transferred through the supply pipes 541, 551 to the cooling devices 54, 55 and then returned into the coolant supply source T through the discharge pipes 542, 552. Thus, the coolant passing through the cooling devices 54, 55 cools the cylinder block 15 in the rotor housing 12. The cooling devices 54, 55, the supply pipes 541, 551 and the discharge pipes 542, 552 cooperate to form a cooling passage through which the coolant for cooling the rotor housing 12 passes. A valve V, which is a part of the cooling passage, is provided between the supply pipes 541, 551 and the coolant supply source T as a means for controllably opening or closing the supply pipes 541, 551. The valve V is provided by an electromagnetically-operated three-way valve which is operable to control the supply of coolant by opening or closing the supply pipes 541, 551.

As shown in FIG. 1, the electric motors MA, MB are electrically connected to an inverter 65 which is in turn electrically connected to a control device 75 which provides command or control signals to the inverter 65. The control device 75 includes a central processing unit (CPU) 75 a and a memory 75 b. The central processing unit 75 a executes various processing according to control programs stored in the memory 75 b. That is, the control device 75 controls the inverter 65 by the central processing unit 75 a according to the control programs stored in the memory 75 b. In the first preferred embodiment, the control device 75 or the central processing unit 75 a, which forms a means for controlling the stopping operation of the vacuum pump of the present invention, controls the stopping operation of the roots pump 10 according to the stopping operation control programs stored in the memory 75 b.

The memory 75 b stores therein data of a predetermined normal rotor speed for the rotors 23 through 32, 51, 52. This normal rotor speed data represents a predetermined rotor speed of the rotors 23 through 32, 51, 52 during the normal operation of the roots pump 10 for gas transferring in a semiconductor manufacturing device. The memory 75 b further stores therein data of a predetermined first preset rotor speed for the rotors 23 through 32, 51, 52 which represents a rotor speed to which the rotor speed is reduced from the aforementioned normal rotor speed in performing the pump stopping operation control.

The above first preset rotor speed is set below a predetermined second preset rotor speed for rotors 23 through 32 and the rotors 51, 52 which is also stored in the memory 75 b. The second preset rotor speed represents such a rotor speed that makes possible preventing rapid reduction of the clearances between the inner surfaces of the pump chambers 39 through 43 and the rotors 23 through 32, 51, 52 facing such inner surfaces without increasing the temperature of the rotor housing 12 during the rotation of the rotors 23 through 32, 51, 52.

The memory 75 b still further stores therein data of a predetermined increased rotor speed for the rotors 23 through 32, 51, 52. The increased rotor speed data represent a rotor speed increased from the aforementioned first preset rotor speed in performing the pump stopping operation control. The increased rotor speed is set below the above predetermined second rotor speed. In the present first preferred embodiment, the rotors 23 through 32 are rotated at substantially the same speed as the electric motor MA, and the rotors 51, 52 rotated at substantially the same speed as the electric motor MB. That is, the rotation speeds of the rotors 23 through 32 and rotors 51, 52 are the same as that of the electric motors MA, MB. The inverter 65 uses an alternating-current power supply 77 as a power source based on the command control of the control device 75 and is operable to control the electric motors MA, MB in accordance with the above rotor speed data and changes the rotation speeds of the rotors 23 through 32 and rotors 51, 52 accordingly.

A temperature sensor S is provided for detecting the temperature of the rotor housing 12 of the multistage roots pump 11A. The temperature sensor S is located at a position on the outer periphery of the pump chamber 43 which is closest to the outlet 171 and has highest temperature among the pump chambers 39 through 43. The temperature sensor S is a detection means which generates to the control device 75 a detection signal when the rotor housing 12 reaches a predetermined temperature during operation of the multistage roots pump 11A. As shown in FIG. 2(a), the valve V is electrically connected to the control device 75 which controls the operation of the valve V.

FIG. 3 is a graph showing changes over time of the rotation speeds of the rotors 23 through 32 and rotors 51, 52 and the temperature of the rotor housing 12 of the multistage roots pump 11A in controlling the stopping operation of the roots pump 10. The horizontal axis in the graph of FIG. 3 shows the elapse of time during the operation of the roots pump 10 and a part of the horizontal axis shows the elapse of time in controlling the operation of the roots pump 10. The vertical axis shows the rotation speed of the rotors 23 through 32, 51, 52 and the temperature of the rotor housing 12.

Bold line G1 in the graph shows the temperature change of the rotor housing 12 in the multistage roots pump 11A and dash-dot line G2 shows the change of rotation speed of the rotors 23 through 32 in the multistage roots pump 11A. Chain double-dot line G3 in the graph shows the change of the rotation speed of the rotors 51, 52 in the single stage roots pump 11B.

The following will describe the method for controlling the stopping operation of the roots pump 10 with reference to the graph in FIG. 3. In the roots pump 10, the single stage roots pump 11B is provided to assist the multistage roots pump 11A in transferring gas and, therefore, it is free from engagement of solid products. In the multistage roots pump 11A, on the other hand, the pressure and the thermal expansion of the casing and pump rotors are increased in accordance with gas transfer from the pump chamber 39 to the pump chamber 43. Therefore, solid products tend to be held in the clearance when the roots pump operation is stopped and the casing and pump rotors of the multistage roots pump 11A contracted. The method of controlling the stopping operation to prevent solid products from being held in the clearance will now be described with reference to the multistage roots pump 11A. The rotary shafts 19, 20, rotors 23 through 32, rotor housing 12 and rear housing 14 in the multistage roots pump 11A are thermally expanded by heat generation during normal operation. Because the rotary shafts 19, 20 are higher in temperature and more expanded, the clearances between the rear side surfaces of the rotors 23 through 32 and their adjacent opposite partition walls 16 and between the rear side surface of the rotor 32 and the rear housing 14 are larger than those when the roots pump is in a stopped state. The valve V is fully closed in normal operation of the roots pump 10.

In normal operation of the roots pump 10, the control device 75 causes the rotors 23 through 32 to be rotated at the normal rotor speed in accordance with the normal rotor speed data, as indicated by dash-dot line G2. In FIG. 3 and the rotors 51, 52 to be rotated at the normal rotor speed in accordance with the normal rotor speed data, as indicated by chain double-dot line G3 in FIG. 3. When stopping the gas transferring operation of the roots pump 10, a pump switch (not shown) is turned off and a pump-stop command signal for stopping the roots pump 10 is transmitted to the control device 75, accordingly. In response to this pump-stop command signal, the control device 75 causes the valve V to fully open. As a result, coolant is supplied from the coolant supply source T and flows In the cooling passage, which is formed by the cooling devices 54, 55, the supply pipes 541, 551 and the discharge pipes 542, 552, for cooling the rotor housing 12.

The control device 75 causes the rotation speed of the rotors 23 through 32, 51, 52 to be lowered from the normal rotor speed to a level below the second preset rotor speed in accordance with the second preset rotor speed data and the first preset rotor speed data. At the time, the control device 75 causes the rotors 23 through 32 in the multistage roots pump 11A to be rotated at the first preset rotor speed and the rotors 51, 52 in the single stage roots pump 11B to be stopped temporarily. Then, the rotors 23 through 32 are rotated at a lower speed than in normal operation and the temperature of the rotor housing 12 is gradually lowered as indicated by bold line G1 in FIG. 3. The rotor housing 12, the front housing 13 and the rear housing 14 in the multistage roots pump 11A which had been thermally expanded is gradually contracted in accordance with the decreasing rotor speed.

Thus, the clearances between the rotors 23 through 32 and the inner surfaces of the pump chamber 39 through 43 facing the rotors 23 through 32 are relatively narrow as compared to those during normal operation, but they are relatively large as compared to those when the roots pump operation is stopped. Thus, if any solid product enters into the clearance, it is removed by the rotors 23 through 32 then rotating at a relatively low speed.

While the rotors 23 through 32 are being rotated at the first preset rotor speed, the control device 75 control the operation of the rotors 23 through 32 in such a way that the rotor speed of the rotors 23 through 32 are rapidly increased and reduced alternately for a plurality of times at a predetermined interval in accordance with the increased rotor speed data. In other words, while the rotors 23 through 32 are being rotated at the first preset rotor speed, the speed of the rotors 23 through 32 is intermittently increased in the range below the second preset speed and such repeated and rapid increase in speed of the rotors 23 though 32 helps to remove solid products. In accordance with the increased rotor speed data, the control device 75 causes the rotors 51, 52 of the single stage roots pump 11B to rapidly increase their speed at a predetermined interval immediately before rapidly increasing the rotor speed of the rotors 23 through 32 of the multistage roots pump 11A. By so doing, the required driving torque of the electric motor MA for the multistage roots pump 11A may be reduced.

As indicated by dash-dot line G2 in FIG. 3, the rotor housing 12, the front housing 13 and the rear housing 14 are gradually cooled while the rotors 23 through 32 are rotating at the first preset rotor speed and increasing their speed to the increased rotor speed. When the temperature of the rotor housing 12 is decreased to a predetermined temperature which is lower than a temperature of the rotor housing 12 during normal operation of the rotors 51, 52, the temperature sensor S generates a detection signal to the control device 75. When the rotor housing 12 reaches the predetermined temperature, solid products in the clearances have been almost removed and contraction of the rotor housing 12, the front housing 13, the rear housing 14 and rotors 23 through 32, 51, 52 has been stopped, so that the clearances will not become narrower anymore. Responding to the detection signal from the temperature sensor S, the control device 75 stops the electric motors MA, MA and the rotation of the rotors 23 through 32, 51, 52, with the result that the operation of the roots pump 10 is stopped.

The first embodiment has the following advantageous effects.

(1) When the operation of the roots pump 10 (or the multistage roots pump 11A) is stopped with simultaneous stop of gas transferring, the control device 75 causes the rotation speed of the rotors 23 through 32 to be decreased below the second preset rotor speed which is lower than the normal rotor speed during normal operation of the roots pump 10. The control device 75 maintains the rotors 23 through 32 at a relatively low speed which is lower than the second preset rotor speed. The rotor housing 12, the front housing 13, the rear housing 14 and the rotors 23 through 32 in the multistage roots pump 11A which had been thermally expanded during normal operation are cooled while maintaining a slightly thermally expanded state. Thus, the clearances between the rotors 23 through 32 and their facing inner surfaces of the pump chambers 39 through 43 are made larger than those when the roots pump is in a stopped state, so that the rotating rotors 23 through 32 can remove any solid products accumulated in the clearances. Solidified or liquefied products are prevented from getting into the clearances between the adjacent opposite rotors 23 through 32 which will not become narrower when the roots pump is stopped and being fixed into the clearances between the adjacent opposite rotors 23 through 32. Therefore, the method of controlling the stopping operation of the roots pump 10 can successfully prevent solid products from being held in the clearances.

As a result, the roots pump 10 can be restarted with no solid product present in the clearance and there is no need to manually apply a large torque to the rotary shafts 19, 20 for breaking solid products by rotating the rotors 23 through 32 for many times in the background art. Thus, restarting of the roots pump 10 requires no preliminary work of removing solid products from the clearances, so that gas transferring operation can be initiated immediately after a restart of the roots pump. In addition, since there is no need to manually apply a large torque to the rotary shafts 19, 20 as in the background art, the rotary shafts 19, 20 need not be made rigid enough to resist the large torque, so that the rotary shafts 19, 20 or the roots pump 10 can be downsized.

(2) In response to a pump-stop command signal, the control device 75 is operated to fully open the valve V, thereby allowing the coolant to circulate for cooling the rotor housing 12. Thus, the rotor housing 12 is cooled more efficiently by the coolant as compared to before the valve V is fully opened. Although the rotor housing 12 contracts by cooling, the rotors 23 through 32 keep rotating at a speed under the second preset rotor speed, so that the clearances between rotors 23 through 32 and their facing pump chambers 39 through 43 are larger than those when the roots pump is stopped and, therefore, solid products are prevented from being held in the clearances. Cooling the rotor housing 12 by circulating coolant, the time before the rotor housing temperature is reduced to a predetermined level can be shortened and also the time that is required before the roots pump 10 is stopped while solid products being removed from the clearances can be reduced.

(3) When the temperature of the rotor housing 12 reaches a predetermined temperature and the temperature sensor S generates a detection signal, accordingly, the control device 75 stops the rotations of the rotors 23 through 32. According to this method of controlling the stopping operation, the rotors 23 through 32 are stopped when the temperature of the rotor housing 12 becomes relatively low and the clearances between the rotors 23 through 32 and their facing inner surfaces of the pump chambers 39 through 43 are almost completely contracted. With the clearances thus contracted, the clearances are further narrowed and solid product will not be held in the clearances. Thus, the rotation of the rotors 23 through 32 cannot remove solid products, so that the driving force of the electric motor MA rotating the rotors 23 through 32 is only wasted. Controlling the stopping operation of the rotors 23 through 32 in accordance with the temperature of the rotor housing 12, power consumption of the motors can be restrained.

(4) The control device 75 controls the operation of the rotors 23 through 32 in such a way that their rotor speed is rapidly increased while they are being at the first preset rotor speed that is lower than the second preset rotor speed. Accordingly, the force then applied to solid products from the rotors 23 through 32 is increased rapidly, so that solid products in the clearances can be removed efficiently as compared to the case where the rotors 23 through 32 are rotated at a constant rotation speed.

(5) The control device 75 controls the operation of the rotors 23 through 32 such that they are rotated at the first preset rotor speed which is lower than the second preset rotor speed and the rotor speed is rapidly increased under the second preset rotor speed. Thus, in controlling the stopping operation, the rotation speed of the rotors 23 through 32 is maintained below the second preset rotor speed. This prevents the temperatures of the rotor housing 12 and rotors 23 through 32 from becoming higher than necessary and the length of time in which the temperature of the rotor housing 12 reaches the predetermined temperature is restrained to necessity minimum.

A second preferred embodiment of the present invention will be described with reference to FIG. 4, in which the present invention is applied to a method for controlling the stopping operation of a roots pump as a vacuum pump which transfer gas in a semiconductor manufacturing device and also to a device for practicing the method. The second embodiment differs from the first embodiment in that the method of controlling the stopping operation uses only the multistage roots pump 11A. Those descriptions which have been already made with reference to the first embodiment will be omitted.

In the second preferred embodiment, controlling of the stopping operation of the multistage roots pump is performed in accordance with the graph shown in the FIG. 4. The graph in FIG. 4 shows changes over time of the rotation speeds of the rotors 23 through 32 and the temperature of the rotor housing 12 in controlling the stopping operation of the multistage roots pump 11A. The horizontal axis represents the elapsed time in operation of the roots pump, a part of which represents the elapsed time in controlling the stopping operation. The vertical axis of the graph represents the rotation speed of the rotors 23 through 32 and the temperature of the rotor housing 12.

Bold line G1 in the graph of FIG. 4 shows a change of the temperature of the multistage roots pump 11A and dash-dot line G2 shows a change of the rotation speed of the rotors 23 through 32 in the multistage roots pump 11A.

According to the second preferred embodiment, the control device 75 lowers the rotation speed of the rotors 23 through 32 from a normal rotor speed to first preset rotor speed that is below a second preset rotor speed and then keeps the rotors 23 through 32 rotating at the constant first preset rotor speed without rapidly increasing the rotor speed at a predetermined interval as in the first preferred embodiment. Therefore, the second preferred embodiment has the effects similar to the effects (1) through (3) of the first preferred embodiment.

The present invention may be practiced in other various modifications as exemplified below.

In the above preferred embodiments, the control device 75 may control in such a way that the rotation speed of the rotors 23 through 32 of the multistage roots pump 11A is rapidly decreased at a predetermined interval while the rotors 23 through 32 are being rotated at the first preset rotor speed, with the valve V then fully closed to stop supplying of coolant to the pipes 541, 551. By thus controlling, contraction of the rotor housing 12 and rotors 23 through 32 is delayed and solid products are removed by changing the rotor speed.

In the above preferred embodiments, controlling may be performed in such a way that rotation speed of the rotors 23 through 32 of the multistage roots pump 11A are either increased or decreased rapidly only once while the rotors 23 through 32 are being rotated at the first preset rotor speed.

In the above preferred embodiments, while the rotors 23 through 32 of the multistage roots pump 11A are being rotated at the first preset rotor speed, the control device 75 may be operable to control in such a way that the rotation speed of the rotors 23 through 32 is increased when the current supplied to the inverter 65 is increased so that first preset rotor speed is maintained. Any increase in the current supplied to the inverter 65 means that any part of the rotors 23 through 32 is prevented from rotating by the presence of solid products. Therefore, increasing the rotor speed when the current to the inverter is increased, solid products that prevent rotating of the rotors 23 through 32 can be broken and removed.

In the above preferred embodiments, the control device 75 may not control the operation of the valve V.

The controlling method of the present invention may be applied to a vacuum pump which includes the single stage roots pump 11B.

The controlling method of the present invention may be applied also to a vacuum pump having a screw rotor as a gas transferring body. The clearance between the screw rotors engaged with each other is larger by being thermally expanded during normal operation and narrower when the roots pump is stopped. The solid products are prevented from getting into and being held in the clearances between screw rotor and opposite housing as well as between screw rotors.

In the above preferred embodiments, it may be so arranged that decreasing of the rotor speed from the normal speed to the second preset rotor speed may take place gradually.

In the above preferred embodiments, the rotation speed of the electric motor MA, MB may be different from that of the rotors 23 through 32, 51, 52 by the shaft coupling 44.

The cooling passage may be formed within the thickness of the rotor housing 12.

Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims. 

1. A method for controlling the operation of a vacuum pump when stopping the gas transferring operation thereof, the vacuum pump having a housing in which a pump chamber is formed and a gas transferring body which is rotatably disposed in the pump chamber for transferring gas, comprising the steps of: reducing rotational speed of the gas transferring body to a first preset speed below a second preset speed that is lower than a normal speed of the gas transferring body during normal gas transferring operation of the vacuum pump; maintaining the speed of the gas transferring body below the second preset speed; and stopping the rotation of the gas transferring body when the temperature of the housing reaches a predetermined temperature which is lower than that of the housing during the normal gas transferring operation of the vacuum pump.
 2. The method according to claim 1, wherein the vacuum pump further has a cooling passage through which coolant flows for cooling the housing and the step further includes the additional step of: flowing the coolant in the cooling passage before the speed of the gas transferring body is reduced to the speed below the second preset speed.
 3. The method according to claim 1, wherein the step further includes the additional steps of: rapidly increasing the rotational speed of the gas transferring body in the range below the second preset speed while the gas transferring body is being rotated at the speed below the second preset speed after the reducing the rotational speed of the gas transferring body; and reducing the rotational speed of the gas transferring body below the second preset speed.
 4. A device for controlling the operation of a vacuum pump when stopping the gas transferring operation thereof, the vacuum pump having a housing in which a pump chamber is formed, a gas transferring body rotatably provided in the pump chamber for transferring gas, the device comprising: means for controlling the stopping operation of the vacuum pump; and detection means for detecting the temperature of the housing and generating a detection signal when the temperature of the housing reaches a predetermined temperature which is lower than a temperature of the housing during normal operation of the vacuum pump when gas is being transferred by the gas transferring body; wherein the controlling means is operable to reduce rotating speed of the gas transferring body to a speed below a second preset speed that is lower than a normal speed of the gas transferring body during normal gas transferring operation of the vacuum pump in response to a pump-stop command signal for stopping the gas transferring body and also to stop the rotation of the gas transferring body in response to the detection signal from the detection means.
 5. The device according to claim 4, the vacuum pump further having a cooling passage being formed therein through which coolant for cooling the housing passes and a means for opening and closing the cooling passage, wherein the means for controlling the stopping operation of the vacuum pump is operable to open the means for opening and closing the cooling passage before the rotational speed of the gas transferring body is reduced to the speed below the second preset speed.
 6. The device according to claim 4, wherein the means for controlling the stopping operation of the vacuum pump is operable to rapidly increase the rotational speed of the gas transferring body in the range below the second preset speed while the gas transferring body is being rotated at the speed below the second preset speed after the reducing the rotational speed of the gas transferring body and also to reduce the rotational speed of the gas transferring body below the second preset speed after the rapid increase of the rotational speed. 